Ski belongs to a family of protooncoproteins that transform avian embryo fibroblasts upon overexpression. It is a nuclear protein capable of activating or repressing gene transcription in a context dependent manner. We have shown that Ski can interact with Smad3, an intracellular mediator of transforming growth factor Beta (TGF-Beta) signaling. TGF-Beta potently blocks cellular proliferation; thus components of TGF-Beta signaling are candidates of tumor suppressors. Inactivation of TGF-Beta signal transducers through mutations or activation of oncogenes occurs as a frequent event in the genesis of human cancer. We have found that in response to TGF-Beta signaling, Ski forms a complex with Samd3/4. The resultant protein complex does not impede Smad's DNA binding, however blocks transcription mediated by Smads through recruitment of transcription corepressors. Epithelial cells overexpressing Ski are resistant to growth inhibition by TGF-B, which potentially implicates Ski's mechanism of transformation. More recently, we have found that overexpression of Ski in mammalian cells can lead to impaired regulation of cell cycle and proliferation. In addition, Ski appears to abrogate function of p300, another important tumor suppressor protein acting as an integrator of variety of cellular signaling pathways. Here we propose to determine the relationship between cell cycle and proliferation regulation by Ski and its ability to interact with Smad. We will also determine the molecular mechanisms underlying the regulation of p300 by Ski, and explore the role of Ski in cellular differentiation. These studies will lead to a better understanding of the cellular and molecular contexts for Ski's ability to regulate cell growth and differentiation as well as mechanisms of TGF-Beta signaling, and ultimately will help us to understand molecular mechanisms of tumorigenesis in general.